Not difficult at all.
I once lost 40 lbs within 9 mths by 4 running per week (45 mins@) + one hiking.
I also skiped my lunch from Mon to Friday and eat fruits instead.
Taking food at least 1 hr after exercise (or only drinking water).
It worked, but 138 lbs is probably too thin for a 5'11" male runner.
I quitted smoking more than 2 decades ago, and it's still my best decision I've ever made. Unfortunately the smokers always smoke in public area and leaving their 2nd handed smoke behind!
Not so hard to me. I made up my mind to quit and never touch it again. The smoke became so stink to me afterwards. However never start smoking is 100 % correct.